Ornamental design for a guitar or similar instrument



FIG. 1 is a front, top and left side perspective view of a guitar or similar instrument showing my new design;

FIG. 2 is a bottom plan view;

FIG. 3 is a rear elevational view; and

FIG. 4 is a right side elevational view. 

The ornamental design for a guitar or similar instrument, as shown and described. 